First of all, understand this. Codes are written to ensure safety and functionality first and foremost. Sometimes the code makes sense to anyone that reads it, sometimes it makes little sense and seems inconvenient. That is when we get situations like your's and your daughters plumbing issues.
Most model codes call for a separate trap for a disposer for just this reason. The disposer can push dirty water into the other bowl of the sink where there might be clean food or dishes and contaminate them.
Its not a likely situation, but codes look more at possibilities, not really probabilities. Some local code officials think this such a low probability that they give in to pressure to exempt or cut out this section of the code for single family dwellings, but may still require it for churches, schools, day cares, etc.
The setup you're seeing in your daughter's home is called a continuous end-outlet waste. Its designed for two sink bowls to share a trap. I don't know what your local code calls for, but after one year, I wouldn't think the builder/plumber would have any responsibility to make corrections now.
Continuous end-outlet waste
Take a picture and start a new thread. We can tell you how to add a second trap to the disposer if you want to tackle it. If not, its usually not a lot of bother for a good plumber to change it over.
Or you can check out Kirby Palm's page on a disposer installation
he did. Really good drawings and a nice picture of a disposer on its own trap. He's made the sink drain on one bowl a bit more complicated than need be, but nice work for a DIYer. His installation didn't include a dishwasher drain.